Database systems (DBMS Database Management System) access a stock of data. This stock of data is typically stored on a hard disk. In many cases, the data are also stored in a ROM, as is the case with a voice database T9 for mobile radio telephones, for example. In addition, by way of example, telephone books are stored on optical disks, such as CD-ROMs or DVD-ROMs, to some extent.
However, storing a dynamic database on an optical medium is currently avoided. The reason for this is that the read heads for optical media have very long skip times, particularly when the starting point and the destination of the skip are at a great radial distance from one another, and an optical medium allows only a limited number of rewrite cycles in comparison with a hard disk.
The stock of data in a database is frequently provided with an index for optimizing search operations. There are a number of options for accessing an index:    1. Load the index into the main memory (e.g. DRAM) and access it there,    2. Leave the index on the disk and load it into the main memory when required and use it,    3. Search the index on the disk,    4. Buffer-store the index on a hard disk; load corresponding index into the main memory when required and search it, or    5. Buffer-store the index on a hard disk; search the corresponding index on a hard disk directly when required.
The first variant requires a very large number of expensive main memories. This variant is therefore seldom feasible. The second variant is more realistic, particularly when there are a large number of index lists or indices. The third variant saves main memory, but is also the slowest variant.
The fourth variant is a supporting variant in which appropriate index lists are stored temporarily on a hard disk (if present) in order to ensure faster access. Similarly, the fifth variant makes it possible to implement the search entirely on the hard disk.
Particular restrictions apply to an optical, writable disk in comparison with a hard disk.                An optical disk requires a very long time to skip from an arbitrary sector to another arbitrary sector (up to 1 second).        However, the continuous reading-in of sectors is in orders of magnitude similar to that for current hard disks.        The number of times that it is possible to rewrite to a sector is limited (between 1000 and 100 000 times, depending on media type).        Disks such as DVD and Blu-ray protect sectors by means of “ECC” (Error Correction Code). In this case, 16 (DVD) or 32 (Blu-ray) sectors are combined to form a block (ECC block) and are provided with a corresponding additional code. This means that an ECC block can only ever be read and written in full, regardless of whether just one byte of a sector is to be accessed or the entire ECC block.        